UFC president Dana White has warned the MMA public that they would be crazy to write off Stephan Bonnar in the UFC 153 main event against Anderson Silva, pointing out that the underdog is enormous in size compared to his rival.

Bonnar stepped up when many others would not to take on UFC middleweight champion Silva on October 13 in Brazil. Nobody has ever beaten Silva in the UFC, and he is tipped to annihilate Bonnar in little over a week's time.

However, Silva is stepping up to light-heavyweight for the third time in his career, having previously beaten James Irvin and Forrest Griffin. He will give away a significant height and reach advantage to Bonnar, who is a naturally big 205-pounder.

Crucially, Bonnar has never been knocked out in his MMA career, which may allow him to get close enough to Silva to force a ground battle. If he can take it there, his ground skills are extremely dangerous.

"He's huge, he's going to be so much bigger than Anderson in this fight that it's going to be crazy," White told ESPN.

"People don't realise what a huge Rocky story this is. Stephan has been so close for so long. He's been waiting for a big fight to come up and then, boom, this comes up.

"Stephan called us to ask for this fight. He's on a four-fight win streak and I know how much this means to him. He's going after it and this is the biggest fight of his life.

"Anderson has fought at 205 before, you saw what he did with Forrest, but this is a completely different fight. It's a dangerous fight for Anderson."

Victory for Silva would be his 16th in the UFC, and White has long argued that The Spider is the greatest fighter of all time. Asked if he feels like a fan of the Brazilian, White did not even try to hide his partiality.

"I'm a huge Anderson Silva fan, he's the greatest of all time, what he does with his hands and feet is amazing," White said. "He does things to people that other people can't. He's awesome and I'm a huge fan."